diff --git a/.nvmrc b/.nvmrc
new file mode 100644
index 00000000..2bd5a0a9
--- /dev/null
+++ b/.nvmrc
@@ -0,0 +1 @@
+22
diff --git a/CLAUDE.md b/CLAUDE.md
index 7546890e..3260d41c 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -63,8 +63,10 @@ The numbered filenames define concatenation order:
 
 ## Key Constraints
 
-- **Node 22** is required (matches CI). Use `nvm use` if you have nvm configured.
+- **Node 22** is required (matches CI and `.nvmrc`). Run `nvm use` to select it. Older
+  versions fail the build: several dependencies (`del`, `postcss-custom-properties`, …) are
+  ESM-only and the gulpfile loads them with `require()`, which only works on Node ≥ 22's
+  support for `require()`-ing ES modules.
 - **npm ≥ 11.11.0** is required.
 - `js/vendor/` files are excluded from ESLint — do not add linting rules targeting them.
 - CSS custom properties are intentionally left un-prefixed (stylelint rule disabled).
-- The `del` and `chokidar` packages are ESM-only; the gulpfile handles this with dynamic `import()`.